When Does 2+2=3? Exploring a Mechanism for Negative Priming of Soil Organic Matter by Black Carbon Dissolved Organic Matter.

Black carbon (BC) plays a critical but poorly understood role in the global carbon cycle. Because it is a highly stable form of carbon compared to the original biomass from which it is produced, its production and management has been proposed as a technology for reducing atmospheric CO₂ stocks, which contribute to climate change. However, uncertainties remain regarding the effect of its application on the decomposition rate of existing soil organic carbon (SOC) stocks. BC additions may cause SOC to decompose at a different rate than it would without the BC application – an effect termed "priming". We aim to determine whether the relative amounts of labile SOC and dissolved BC (DBC) can be used to predict the magnitude and direction of "priming" in a soil system. This study will investigate potential mechanisms for short-term priming in a 5-week incubation study. The BC is produced at 325°C from ¹³C-labelled maple twigs, which will facilitate distinguishing the source of soil CO₂ emissions using an isotopic partitioning approach. "Labile" SOC levels will be adjusted by incubating soils for varying durations. DBC levels will be adjusted by extracting DBC from bulk BC and then returning it to the bulk BC at varying rates. We predict that the greatest negative priming effects will be seen where there is low SOC and high DBC, as DBC may be used preferentially as a substrate by microbes in SOC-limited soils. This research will further understanding of the critical interactions between BC and SOC while also informing policy decisions regarding BC production and application to soils for carbon sequestration.